6

High peak currents associated with capacitive loads ne-

cessitate careful grounding techniques. Timing and by-

pass capacitors should be connected close to pin 5 in a

single point ground. The transistor and 5k potentiometer

are used to sample the oscillator waveform and apply

an adjustable ramp to pin 3.

Shutdown of the UC1842 can be accomplished by two

methods; either raise pin 3 above 1V or pull pin 1 below

a voltage two diode drops above ground. Either method

causes the output of the PWM comparator to be high

(refer to block diagram). The PWM latch is reset domi-

nant so that the output will remain low until the next

clock cycle after the shutdown condition at pin 1 and/or

3 is removed. In one example, an externally latched

shutdown may be accomplished by adding an SCR

UVLO threshold. At this point the reference turns off, al-

lowing the SCR to reset.

UC1842/3/4/5

UC2842/3/4/5

OUTPUT SATURATION CHARACTERISTICS

ERROR AMPLIFIER OPEN-LOOP

FREQUENCY RESPONSE

OPEN-LOOP LABORATORY FIXTURE

SHUT DOWN TECHNIQUES